Imagers, including complimentary metal oxide semiconductor (CMOS) imagers and charge-coupled devices (CCD), may be used in digital imaging applications to capture scenes. An imager includes an array of pixels. Each pixel in the array includes at least a photosensitive element for providing a signal having a magnitude proportional to the intensity of light incident on the photosensitive element. When exposed to incident light to capture a scene, each pixel in the array outputs a signal having a magnitude corresponding to an intensity of light at one point in the scene. The signals provided by all of the photosensitive elements may be processed to form an image representing the captured scene.
In one imager incorporating a 4T pixel structure, each pixel further includes a floating diffusion region for temporarily storing a signal from the photodiode, a reset transistor for resetting the floating diffusion region and the photodiode, a row select transistor for selecting a pixel for read out and a source follower transistor for reading out the level of the pixel.
In another imager incorporating a 4T pixel structure, each individual pixel may include only a photodiode and a transfer gate. The remaining elements may be shared amongst a plurality of pixels. That is, a plurality of pixels may share a common floating diffusion, reset transistor, source follower transistor and row select transistor. This is commonly known as pixel multiplexing. Some common multiplexed arrangements include 2-way, 3-way and 4-way multiplexed pixel arrays wherein 2 pixels, 3 pixels or 4 pixels, respectively, share the common transistors.
Pixel multiplexing may be desirable because it reduces the average number of transistors per pixel. For example, in a 2-way multiplexed pixel array, the average number of transistors per pixel is reduced from 4 transistors to 2.5 transistors. By way of another example, for the 4-way multiplexed pixel array, the average number of transistors per pixel is further reduced to 1.75 transistors per pixel. In this way, multiplexing pixels may serve to reduce the size of each pixel.